Insulation for direct current (DC) transmission systems is important for the reliability of a transmission system. The reliability depends on the material used for covering the conductor layers. The geometry of the insulation material around the transmission system is also important.
The amount of power that can be delivered by a DC cable has increased dramatically in the past decades. Further increasing the amount of power that can be delivered by a DC cable can be achieved in several ways as described by Nordberg et al., Cigre, Session 2000, 21-302. Examples mentioned are increasing the size of the conductor or alternatively increasing the voltage. The latter has the benefit of lower power losses but necessitates an increase in the thickness of the insulation in general. This will increase the cables' size and weight. An alternative solution is to increase the maximum allowed conductor temperature or to increase the dielectric strength of the insulation material.
New insulation liquids have been developed, such as gelling liquids described in U.S. Pat. No. 6,383,634, to allow an increase in conductor temperature.
Laminated insulation materials have been developed to increase the dielectric strength of the insulation material. As explained by Hampton R., IEEE Electrical Insulation Magazine, Vol 24, No 1, 2008, page 5, important parameters for the provision of a reliable DC insulation material are electrical resistivity at a range of stresses and temperatures, DC breakdown performance, sensitivity to electrical aging and space charge development. Resistivity is dependent on DC stresses and temperatures as well as on the thickness of the insulation material, whereby the resistivity decreases with increased stress and temperature. Electrical charges that become trapped within the insulation material (space charge) will also have an effect on the electrical stress performance of the material. The breakdown strength may decrease with time of applied DC stress due to such space charges. The geometry of a transmission system such as a cable, cable joints, buses and the like, and the distribution of the temperature are further critical factors for the reliability of the DC transmission system. Hampton also explains the advantage of a homogenous insulation layer and mentions that a laminated insulation system may be a source for inhomogeneity, which in turn may affect the quality of the insulation material. Leakage of current should preferably be prevented. If leakage becomes too high, dielectric heating may occur. This condition may result in melting.
JP 10 283852 describes an insulation material for use in a direct current high viscosity oil impregnated power cable, whereby the insulation material comprises multilayer of paper and laminated paper sheets.
WO2011/073709 describes a high voltage direct current (HVDC) cable comprising an insulation layer of laminated polypropylene (PP)/Kraft paper. The insulation layer has a constant thickness over the entire insulation layer. The invention relates to delamination of the insulation layer during impregnation with an impregnation fluid having a medium viscosity of at least 1000 cSt at 60° C. and an air impermeability of at least 100000 Gurley sec−1. This problem is solved by using special paper in the insulation laminate.
U.S. Pat. No. 7,943,852 describes a superconducting cable that can be used in both DC and alternating current (AC) cables. The cables are housed in a heat-insulated pipe filled with a coolant. The resistivity of the laminated polymer (PP)/paper insulation material can be varied by varying the density, or by adding dicyandiamide to the paper, or by varying the thickness ratio of polymer to paper in the laminate. The insulation layer has a low resistivity on the inner part close to the conductor layer and a higher resistivity at increasing radial distance from the conductor layer. In the examples, Kraft paper is positioned around the conductor, while laminated polymer/paper is used as insulation material in the rest of the insulation layer. This laminated insulation layer comprises material having an increasing resistivity at increased radial distance so that the cable also has excellent AC electrical properties.
U.S. Pat. No. 6,399,878 describes insulation material for DC cables that may comprise three different parts, whereby the inner and outer part closest to the semiconductive layers contain paper that has a low resistivity. The middle insulation part comprises laminated polymer/paper material having higher resistivity. This layer may be divided in different parts, whereby the different parts have different polymer/paper ratios and whereby the ratios decrease at increasing radial distance from the inner conductor layer. (FIGS. 8a, 8b, 13 and 14) The resistivity in the middle layer thus decreases at increasing radial distance. The insulation material is impregnated with a medium viscosity oil having a viscosity from 10 centistokes and less than 500 centistokes (cSt) at 60° C.
U.S. Pat. No. 6,207,261 describes a laminated polymer/paper insulation material for DC cables, which is impregnated with a medium viscosity fluid. The thickness of the laminate may be varied by varying the thickness of the paper or the polymer. Nothing is mentioned about variation of thickness of the laminated material within one cable. After lamination, the laminate is being calendered or supercalendered. The paper in the laminate has one smooth and one rough surface.
EP 875907 describes insulation material comprising paper at the inner and outer part of the insulation layer, which paper material has low resistivity. The middle part comprises laminated polymer/paper material having higher resistivity. The thickness of the paper may be varied to change the resistivity. The aim of the invention is to provide insulation material having a resistivity between 0.1 ρ0 and 0.7 ρ0, where ρ0 is the resistivity of the normal Kraft paper, over the whole temperature range. This may be achieved by varying the quality of the materials, or using additives such as amine or cyanoethylpaper.
Hata R. SEI Technical review, 62, June 2006, page 3, describes solid DC submarine cable insulated with polypropylene (PP) laminated paper, whereby the inner part of the insulation layer in the vicinity of the conductor layer comprises paper, which is covered by a layer of laminated PP forming the middle part of the insulation material, which is subsequently covered with paper which forms the outer part of the insulation layer.
U.S. Pat. No. 3,987,239 describes insulation material, whereby the electrical stress distribution in a high voltage system is improved by providing insulation material comprising different parts located at different radial distances from the conductor layer. The different parts may comprise the same or different insulation material. The effect of the arrangement of layers is that the resistivity gradient in the insulation material from the inner part to the outer part of the insulation layer is as flat as possible. FIG. 9 in U.S. Pat. No. 3,987,239 shows that the resistivity is flat at the inner part of the insulation layer and then decreases at increasing radial distance from the conductor layer. The plastic material used has an E-stress below 22 kV/m. Modern insulation materials have an E-stress above this value.
U.S. Pat. No. 4,075,421 describes insulation paper, whereby the resistivity in the most inner part is higher compared to the resistivity in the outer part of the insulation layer.
A limiting factor in the development of DC transmission systems, especially cable joints and cable terminations, is the insulation breakdown strength. Experiments have shown that the breakdown location in a cable is often started from the semiconductive layer/insulation layer interface.
There is a need for insulation material, whereby the resistivity is lowered at locations close to the inner and outer semiconducting layers. There is a need for an improved resistivity control in the insulation material, especially at these locations. By improving the electrical field stress distribution, the breakdown stress of the insulation material can be improved.
Although many improvements have been made to laminated insulation materials for DC transmission systems, there is still a need for improving the electrical performance, increase the transmission capacity, improve the reliability, decrease aging and manufacturing costs for insulated transmission systems. With regard to high and ultra high voltage (UHV) DC and (U)HVDC for mass-impregnated non-draining (MIND) transmission systems there is a need for improved resistivity control over the entire insulation layer, especially with regards to insulation materials impregnated with high viscosity fluids.